The present disclosure relates generally to data processing and relates, more particularly to processing input/output (I/O) operations in a channel using a control block.
In the Enterprise Systems Architecture/390 implemented by IBM® System z, input/output (I/O) operations are initiated via a “Start Subchannel” instruction. This instruction designates a subchannel (i.e., an internal representation of an I/O device to be used for the operation) and an Operation Request Block (ORB), which, in turn, specifies the address of a Transport Control Word (TCW) or Channel Command Word (CCW) channel program. These specify the list of commands to be executed by the device.
In executing the Start Subchannel instruction, a processing system copies the ORB information into the subchannel and passes the operation to a System Assist Processor (SAP). The SAP selects a channel to perform the operation, and places a channel communication area (CCA) on the work queue for the selected channel in a base hardware system area (HSA).